Electrical filtering apparatus



Dec. 9, 1930.

P. H. GEIGER ET AL ELECTRICAL FILTERING APPARATUS Filed Oct. 7, 1925 2.Ewnbbeb afqoaau I To Source of Alien eat Current HQEQQ Q Patented Dec.9, 1930 UNITED STATES PATENT OFFlCE PAUL H. GEIGER, OF WILKINSBURG, ANDLABS O. GRONDAHL OF PITTSBURGH, PENN- SYLVANIA, ASSIGNORS TO THE UNIONSWITCH & SIGNAL COMPANY, OF SWISSVALE,

PENNSYLVANIA, A CORPORATION OF PE NN SYLVAN IA ELECTRICAL FILTEBINGAPPARATUS Application filed October 7, 1925. Serial No. 61,013.

Our invention relates to electrical filtering apparatus, andparticularly to apparatus interposed between a source of pulsatingcurrent and a load for reducing the pulsations in the current suppliedto the load.

We will describe one form of filtering apparatus embodying ourinvention, and will then point out the novel features thereof in claims.

In the accompanying drawing, Fig. 1 1s a diagrammatic view illustratingone form of filtering apparatus embodying our invention. Fig. 2 is adiagram showing, graphically, one characteristic of a portion of theapparatus illustrated in Fig. 1.

Referring first to Fig. 1, the apparatus comprises a source of pulsatingcurrent here shown as a rectifier R supplied with energy from thesecondary 2 of a transformer T. The primary 1 of the transformer issupplied with alternating current from a source not shown in thedrawing. The rectifier R may be of any suitable form, and as here shownit is made up of four rectifying elements 3, each of which, may comprisea metal having a compound formed thereon. One form of element iscomposed of copper having cuprous oxide formed thereon, and has theproperty of offering a very much higher resistance to current flow fromthe copper to the oxide, than from the oxide to the copper. Rectifierelements of this type are shown and described in an application forLetters Patent of United States by Lars O. Grondahl, Serial #1,111,filed Jan. 7 1925, for Uni-directional current carrying devices.

The energy supplied by the transformer T is changed by the rectifier Rinto pulsating current which is supplied to a load indicateddiagrammatically at L.

Two reactors A and B are connected in.se ries with the load L, and aresistor K is connected across the rectifier R and the reactor A. Thisresistor K comprises one ormore elements 4, each similar to the elements3 of rectifier R, and connected in series so that current flowingthrough the shunt path Including reactor A and resistor K traverses eachof the elements 4 in its low resistance direction.

We have discovered that uni-directional current carrying devices of thetype disclosed in the patent application mentioned above have thecharacteristic of varying their electrical resistance in response tochanges in the electromotive force applied thereto. Referring now toFig. 2, the curve 5 represents the values of the resistance of one ofthe elements 4 in the low resistance direction for different values ofapplied electromotive force. It will be seen that for a limited range ofapplied electromotive force, represented by the portion of the curve 5indicated at X, small iu creases in the electromotive force applied tothe element cause comparatively large dccreases in the resistance of theelement. The resistor K is made up of such a number of elements 4connected in series that the electromotive force across each unit iswithin this limited range.

Although experiment has shown that de vices similar to those describedin the Gron- I dahl application referred to above are most suitable foruse as elements 4, other substances having similar properties such asgalena or crystalline selenium may be substituted.

In explaining the operation of the apparatus we will first assume thatthe pulsating electromotive force delivered by the rectifier R is at itsmaximum value. The electromotive force applied to the terminals ofresistor K is therefore at its maximum and the resistance of theelements 4 is comparatively low. A considerable current therefore flowsthrough resistor K and causes a large voltage drop in the reactor A. Asa result the electromotive force actually ap plied to the load L is lessthan would be applied thereto if resistor K were omitted from thecircuit. As the electromotive force delivered by the rectifierdecreases, due to the pulsating character of the output of therectifier, the electromotive force applied to resistorrK decreasescorrespondingly. The resistance of the elements 4 therefore increasesand the resulting decrease in the current through the resistor K isgreater than would be the case if the resistor were of usual resistancematerial. Due to the lower value of the current through the reactor A,

v the voltage drop through this reactor detends to decrease this energyis returned to the circuit and tends to maintain constant the currentthrough, the reactor, and hence the current through the load L.

It will therefore be seen that the reactors A and B and the resistor Kcomprise filtering apparatus which tends to smooth out the pulsations inthe current supplied to the load L and to maintain this current at asteady value.

One advantage of our invention is that because of the variations in theresistance of the elements 4; in response to changes in appliedelectromotive force, the power consumed in the branch circuit throughthe resistor K is but a small fraction of the power which would bewasted by an ordinary resistor. That isto say, with apparatus embodyingour invention, much more effective filtering can be accomplished, for -agiven amount of wasted power, than with filtering ap aratus hithertoknown.

lthough we have herein shown and described only one form of filteringapparatus embodying our invention, it is lfilClGIStOOd that variouschanges and modifications may be made therein within the scope of theappended claims without departing from the spirit and scope of ourinvention.

Having thus described our invention, what we claim is:

1. A filter for use with a source of pulsating current and a load,comprising a plurality of elements connected in series across the sourcein parallel with the load, each such element exhibiting a considerablevariation in resistance in response to a comparatively small change inthe electromotive force applied thereto over a limited range of suchelectromotive force, and the number of such elements being such that theelectromotive force across each such element is within said limitedrange.

2. A filter for use with a source of pulsating current and a loadcomprising a plurality of elements connected in series across the sourcein parallel with the load, each such element exhibiting a considerablevariation in resistance in response to a comparatively small change inthe electromotive force applied thereto over a limited range of suchelectromotive force, and the number of such elements being such that theelectromotive force across each such element is within said limitedrange, and a reactor interposed between the source and the load.

3,; A filter for; use with a source of pulsating current and a loadcomprisinga plurality of elements connected in series across the sourcein parallel with the load, each such element exhibiting a considerablevariation in resistance in response to a comparatively small change inthe electromotive force applied thereto over a limited range of suchelectromotive force, and the number of such elements being such that theelectromotive force across each such element is within said limitedrange, a reactor interposed between the source and one terminal of saidelements, and a second reactor interposed between said one terminal ofsaid elements and the load.

In testimony whereof we afiix our signatures.

PAUL H. GEIGER. LABS O. GRONDAHL.

